Latent Heat Flux Measurements Research Articles

Remote sensing estimates of actual evapotranspiration in an irrigation district

Accurate estimates of the spatial distribution of actual evapotranspiration (AET) are useful in hydrology but can be difficult to obtain. Remote sensing provides a potential capability for routinely monitoring AET by combining remotely sensed surface temperature and vegetation cover observations with near surface meteorological data in a surface energy balance model. Results from two different energy balance models are compared to airborne and ground measurements of surface energy fluxes over an irrigation district in northern Victoria during January 2003. Ground data collected include eddy correlation measurements of latent and sensible heat fluxes and associated meteorological measurements. The airborne data include eddy correlation measurements of latent and sensible heat fluxes (35 m above ground level), surface temperature transects and normalised difference vegetation index (NDVI) imagery (1 m resolution). Surface temperature and NDVI maps were derived from Landsat ETM+ data and combined with the ground meteorological observations in both a one- and two-source energy balance model. The two models produced similar results over irrigated sites, but large discrepancies were present over sparsely vegetated and bare soil areas. Although both models overestimated the latent and sensible heat fluxes in comparison to the ground and airborne measured fluxes, it was found that the modelled and observed Bowen ratios compared well.

Calibration of a land surface model using multiple data sets

In order to assess performance and to improve predictions, land surface models are routinely calibrated against measurements of either latent heat or sensible heat fluxes. Generally, little regard is given to the multi-output nature of these models, resulting in a model evaluation that is inherently biased towards the calibration variable. In this paper, an assessment strategy that accounts for multiple outputs is explored and an examination of incorporating alternative sources of information to assess performance is undertaken. The benefits of such a multi-objective calibration framework are illustrated through comparison with traditional single objective calibration. Results indicate that combining different observation data streams for calibration purposes assists in producing a more robust process model and provides improved surface flux predictions. Further, the utility of using correlated, if not commensurate, sources of data, is demonstrated through analysis of a time series of surface temperature measurements.

Comparison of Aircraft and Ground-Based FluxMeasurements during OASIS95

Aircraft and ground-based measurements made during the1995 Australian OASIS field campaign are compared. The aircraft data were recorded during low-level flightsat 6 m above ground level and grid flights at altitudes of between 15 and 65 m, allin unstable atmospheric conditions. The low-level flights revealed an inadequate temperaturesensor response time, a correction for which was determined from subsequent work ina wind tunnel. Aircraft and ground-based measurements of mean wind speed, wind directionand air temperature agree to within 0.2 m s-1, 4° and 0.9 °C respectively.Comparisons between aircraft and ground-based observations of the standarddeviations of vertical velocity, horizontal wind speed, air temperature and specifichumidity have slopes of 0.96, 0.97, 0.92 and 0.99 respectively but the observed scatter isroughly twice the random error expected due to the averaging length of the aircraft data andthe averaging period of the ground-based data. For the low-level flights, the ground-basedand aircraft measurements of sensible and latent heat flux show mean differences of 27 and-25 W m-2 respectively, which reduce to 11 and -4 W m-2 respectivelywhen analysis of aircraft data is limited to areas immediately adjacent to the fluxtowers. For the flights at 15 to 65 m above ground level, the mean differences between theground-based and aircraft measurements of sensible and latent heat flux are -22 and-1 W m-2 respectively and these change to -1 and -7 W m-2 respectively oncethe effect of surface heterogeneity is included. Aircraft and ground-based measurementsof net radiation agree to within 6% at one ground-based site but differ by 20% at a second.Aircraft measurements of friction velocity at 6 m above the ground agree well withground-based data, but those from flights between 15 and 65 m above ground level do not.This is because at these heights the aircraft measurements provide the local shear stress,not the surface shear stress. Overall, the level of agreement allows confidence in the aircraftdata provided due care is taken of instrument response times and differences in thesurfaces sampled by aircraft and ground-based systems.

Ultrasonic detection of atmospheric humidity variations

The small variation of the speed of sound in air with water vapor concentration is evaluated as a method of measuring atmospheric humidity. Laboratory acoustic phase measurements were made, using inexpensive piezoelectric transducers and electret microphones with dew point temperature independently monitored in the same enclosure. Phase variations from an acoustic path gave fair agreement with the measured humidity variations, when temperature and cross-wind variations were removed. Thermal stabilization and filtering were necessary to reduce the random phase noise contributions from the detectors, leading to errors in mean vapor pressure of ∼5 mbar at 20 °C. The approach is therefore more suited to determine turbulent humidity fluctuations, for meteorological latent heat flux measurements.

CALIBRATION OF THE PENMAN-MONTEITH METHOD TO ESTIMATE LATENT HEAT FLUX OVER A GRASS CANOPY

A study was performed at the Panguilemo Evapotranspiration Investigation Plot (ETIP) located 11 km north of Talca, Chile (35°23' South and 71° West; 110 m above the sea level) to evaluate latent heat flux by using the Penman-Monteith method. Data collected on a 20 minute time intervals included latent heat flux (LE), net radiation (Rn), soil heat flux (G), air temperature (Ta), relative humidity (RH), and wind speed (V). The performance of the Penman-Monteith equation (LEPM) was tested with latent heat flux measurements from the eddy correlation system (LEED) over a grass canopy under different soil moisture conditions. Results indicate that there was a good agreement between LEPM and LEED with an overall standard error of estimate (SEE) of 45 W m · -2 and an absolute relative error of 6.2 %.

Correcting eddy-covariance flux underestimates over a grassland

Independent measurements of the major energy balance flux components are not often consistent with the principle of conservation of energy. This is referred to as a lack of closure of the surface energy balance. Most results in the literature have shown the sum of sensible and latent heat fluxes measured by eddy covariance to be less than the difference between net radiation and soil heat fluxes. This under-measurement of sensible and latent heat fluxes by eddy-covariance instruments has occurred in numerous field experiments and among many different manufacturers of instruments. Four eddy-covariance systems consisting of the same models of instruments were set up side-by-side during the Southern Great Plains 1997 Hydrology Experiment and all systems under-measured fluxes by similar amounts. One of these eddy-covariance systems was collocated with three other types of eddy-covariance systems at different sites; all of these systems under-measured the sensible and latent-heat fluxes. The net radiometers and soil heat flux plates used in conjunction with the eddy-covariance systems were calibrated independently and measurements of net radiation and soil heat flux showed little scatter for various sites. The 10% absolute uncertainty in available energy measurements was considerably smaller than the systematic closure problem in the surface energy budget, which varied from 10 to 30%. When available-energy measurement errors are known and modest, eddy-covariance measurements of sensible and latent heat fluxes should be adjusted for closure. Although the preferred method of energy balance closure is to maintain the Bowen–ratio, the method for obtaining closure appears to be less important than assuring that eddy-covariance measurements are consistent with conservation of energy. Based on numerous measurements over a sorghum canopy, carbon dioxide fluxes, which are measured by eddy covariance, are underestimated by the same factor as eddy covariance evaporation measurements when energy balance closure is not achieved.

The dynamic role of root-water uptake in coupling potential to actual transpiration

The relationship between actual ( E act) and potential ( E p) transpiration above a grass-covered forest clearing was investigated numerically and experimentally from simultaneous measurements of soil moisture content profiles, mean meteorological conditions, turbulent heat and water vapor fluxes in the atmospheric surface layer, and soil hydraulic properties for two drying periods. The relationship between E act/ E p was found to be approximately constant and insensitive to variability in near-surface soil moisture content. To explore this near-constant E act/ E p, a model that relates potential and actual transpiration and accounts for root-uptake efficiency, potential transpiration rate, and root-density distribution was proposed and field-tested. The total amount of water consumed by the root system was integrated and compared with eddy-correlation latent heat flux measurements (field scale) and total water storage changes (local scale). Model calculations suggested that the deeper and more efficient roots are primarily responsible for the total water loss within the root zone when the near-surface soil layer approaches their wilting point.

Estimation of surface heat flux and an index of soil moisture using adjoint‐state surface energy balance

Estimates of surface heat flux and an index of the surface control over evaporation are made based on radiometric observations of ground temperature. A variational data assimilation approach is used to include surface energy balance in the estimation procedure as a physical constraint (the adjoint technique). This technique formulates the estimation problem as a minimization of ground temperature forecast misfits against observations. The surface energy balance equation is incorporated as a physical constraint. Applications to the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) are presented. The procedure estimates of the surface control over latent heat flux match those values based on independent latent heat flux measurements. The estimates of surface heat flux, when compared with measurements, have a root‐mean‐square error of 20 W m−2. The need to discriminate between soil wetness and aerodynamic contributions to the surface control over evaporation is recognized.

A Remote Sensing Study of the NDVI– T s Relationship and the Transpiration from Sparse Vegetation in the Sahel Based on High-Resolution Satellite Data

This article proposes a new approach for estimation of the transpiration rate in sparse canopies. The method relies on a combination of techniques; some of which having a successful background of solid experimental and theoretical justification, while others having only recently been introduced as promising tools for the extraction of environmental information from satellite data. The transpiration rate (λ E v ) is calculated by applying an energy balance approach to the vegetation component of the land surface: λ E v=R n v−H v , where R n v is the net radiation absorbed by the vegetation, and H v is the sensible heat flux between the leaves and the air within the canopy. R n v is calculated through the use of remote sensing and standard meteorological data by combining a conventional method for estimation of the land surface net radiation with a ground-calibrated function of NDVI (normalized differential vegetation index). H v is assessed as a linear function of the temperature difference between vegetation ( T v ) and the mean canopy air stream ( T 0). Because the surface temperature ( T s ) recorded by satellite contains combined information of both soil and vegetation, T v is evaluated on the basis of the linear NDVI– T s relationship for individual surface types. T 0 is assessed utilizing recent evidence that ( T s−T 0) is linearly related to the difference in surface temperature and air temperature above the canopy ( T s−T a ), with the slope coefficient depending only on canopy structure. The method is tested using remote sensing data ranging from ground-based, airborne, and satellite recordings. The modeled transpiration rates compared well to measurements of sapflow data and latent heat fluxes recorded for a wide range of surface types (agricultural crops, natural vegetation, forest vegetation).

A technique for estimating dry deposition velocities based on similarity with latent heat flux

Field measurements of chemical dry deposition are needed to assess impacts and trends of airborne contaminants on the exposure of crops and unmanaged ecosystems as well as for the development and evaluation of air quality models. However, accurate measurements of dry deposition velocities require expensive eddy correlation measurements and can only be practically made for a few chemical species such as O 3 and CO 2. On the other hand, operational dry deposition measurements such as those used in large area networks involve relatively inexpensive standard meteorological and chemical measurements but rely on less accurate deposition velocity models. This paper describes an intermediate technique which can give accurate estimates of dry deposition velocity for chemical species which are dominated by stomatal uptake such as O 3 and SO 2. This method can give results that are nearly the quality of eddy correlation measurements of trace gas fluxes at much lower cost. The concept is that bulk stomatal conductance can be accurately estimated from measurements of latent heat flux combined with standard meteorological measurements of humidity, temperature, and wind speed. The technique is tested using data from a field experiment where high quality eddy correlation measurements were made over soybeans. Over a four month period, which covered the entire growth cycle, this technique showed very good agreement with eddy correlation measurements for O 3 deposition velocity.

Observations of Ship Tracks from Ship-Based Platforms

Abstract Ship-based measurements in June 1994 provided information about ship-track clouds and associated atmospheric environment observed from below cloud levels that provide a perspective different from satellite and aircraft measurements. Surface measurements of latent and sensible heat fluxes, sea surface temperatures, and meteorological profiles with free and tethered balloons provided necessary input conditions for models of ship-track formation and maintenance. Remote sensing measurements showed a coupling of ship plume dynamics and entrainment into overlaying clouds. Morphological and dynamic effects were observed on clouds unique to the ship tracks. These morphological changes included lower cloud bases early in the ship-track formation, evidence of raised cloud bases in more mature tracks, sometimes higher cloud tops, thin cloud-free regions paralleling the tracks, and often stronger radar returns. The ship-based lidar aerosol measurements revealed that ship plumes often interacted with the over...

Relevance of Surface Energy Budget within Florida Sea-Breeze Front to Cross-Peninsula Rainwater Runoff Gradient

Abstract Analysis of surface latent heat flux measurements taken within the sea-breeze front of the coast of Florida during active thunderstorm periods demonstrates an important effect of the timing of coastal storms on the seasonal surface water budget. Historical records document a systematic cross-peninsula water runoff gradient across Florida, with total runoff greater on the east coast (Atlantic side) than on the west coast (gulf side). This situation persists even though convective rainfall tends to be greater in the summertime on the gulf side. In this paper, the authors examine the effect of the time of day that summer thunderstorms occur at a given location on poststorm evaporation of rainfall and place these effects into the context of the annual runoff at the coasts and seasonal rainfall in order to assess their possible significance. A surface water exchange analysis, based on datasets obtained during the 1991 summertime Convection and Precipitation Electrification Experiment, finds that part ...

An Intercomparison of Techniques to Determine the Area‐Averaged Latent Heat Flux from Individual in Situ Observations: A remote Sensing Approach Using the European Field Experiment in a Desertification‐Threatened Area Data

A knowledge of the area‐averaged latent heat flux 〈λE〉 is necessary to validate large‐scale model predictions of heat fluxes over heterogeneous land surfaces. This paper describes different procedures to obtain 〈λE〉 as a weighted average of ground‐based observations. The weighting coefficients are obtained from remote sensing measurements. The remote sensing data used in this study consist of a Landsat thematic mapper image of the European Field Experiment in a Desertification‐Threatened Area (EFEDA) grid box in central Spain, acquired on June 12, 1991. A newly developed remote sensing algorithm, the surface energy balance for land algorithm (SEBAL), solves the energy budget on a pixel‐by‐pixel basis. From the resulting frequency distribution of the latent heat flux, the area‐averaged latent heat flux was calculated as 〈λE〉 = 164 W m−2. This method was validated with field measurements of latent heat flux, sensible heat flux, and soil moisture. In general, the SEBAL‐derived output compared well with field measurements. Two other methods for retrieval of weighting coefficients were tested against SEBAL. The second method combines satellite images of surface temperature, surface albedo, and normalized difference vegetation index (NDVI) into an index on a pixel‐by‐pixel basis. After inclusion of ground‐based measurements of the latent heat flux, a linear relationship between the index and the latent heat flux was established. This relationship was used to map the latent heat flux on a pixel‐by‐pixel basis, resulting in 〈λE〉 = 194 W m−2. The third method makes use of a supervised classification of the thematic mapper image into eight land use classes. An average latent heat flux was assigned to each class by using field measurements of the latent heat flux. According to the percentage of occurrence of each class in the image, 〈λE〉 was calculated as 110 W m−2. A weighting scheme was produced to make an estimation of 〈λE〉 possible from in situ observations. The weighting scheme contained a multiplication factor for each measurement site in order to compensate for the relative contribution of that site to 〈λE〉. It was shown that 〈λE〉 derived as the arithmetic mean of 13 individual in situ observations leads to a difference of 34% (〈λE〉 = 104 W m−2), which emphasizes the need for improved weighting procedures.

Analysis of Land Surface Heat Fluxes Using the Orthonormal Wavelet Approach

Heat fluxes under unstable atmospheric conditions are measured and analyzed using orthonormal wavelet expansions. Both wavelet and Fourier power spectra display a −1 power law that can be derived from dimensional arguments for latent and sensible heat flux in the turbulent production subrange. The wavelet expansion is used to investigate the spatial structure of the heat fluxes for those scales that exhibit a −1 power law. Dimensionless statistical measures which provide spatial information at different scales are developed and applied to the sensible and latent heat flux measurements. Deviations from Gaussian statistics were observed at the turbulent production subrange. The large flux events (both positive and negative) in the heat flux signals contribute directly to the energy and spatial structure of the −1 power law. The wavelet transform is used to identify the scale of turbulent action directly responsible for the tails observed in the horizontal gradient probability density function of both heat fluxes.

Intra-field variability of scalar flux densities across a transition between a desert and an irrigated potato field

This paper reports on measurements of sensible and latent heat and CO2 fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.

Cloudiness and Marine Boundary Layer Dynamics in the ASTEX Lagrangian Experiments. Part II: Cloudiness, Drizzle, Surface Fluxes, and Entrainment

The Analysis of the Atlantic Stratocumulus Transition Experiment (ASTEX) Lagrangians started in Part I is continued, presenting measurements of sea surface temperature, surface latent and sensible heat fluxes from bulk aerodynamic formulas, cloud fraction, and drizzle rate for the two Lagrangians, mainly using data from horizontal legs flown by the Electra and C130. Substantial drizzle, averaging 1 mm day−1 at the surface, was measured during the first Lagrangian. The surface fluxes increased rapidly as the air mass advected over rapidly increasing SST. Cloud fraction remained high throughout. During the second Lagrangian, drizzle formed in the stratocumulus layer but mainly evaporated in the deep, dry cumulus layer and the subcloud layer before reaching the surface. Stratocumulus cloud cover was thickest when moist air lay above the inversion and then it dissipated to leave only cumuli once dry air advected over the inversion. Three methods are compared for determining entrainment rate (European Centre for Medium-Range Weather Forecasts analyses of mean vertical motion, calculation of a water budget, and the ozone flux–jump method). While all three methods have significant uncertainties, their predictions are all consistent with an entrainment rate of 0.7 ± 0.3 cm s−1 for the first Lagrangian and 0.6 ± 0.3 cm s−1 for the second Lagrangian. Corresponding estimates of the time-dependent horizontal divergence are also presented. Estimates of the cumulus mass flux, internal mixing time, and entrainment dilution time for the boundary layers observed during the two Lagrangians are also presented.

An intercomparison of single and dual-source vegetation-atmosphere transfer models applied to transpiration from sahelian savannah

Three models of the partitioning of net radiation into latent and sensible heat fluxes over Sahelian savannah are described. Each model has a different configuration of stomatal and aerodynamic resistances. Their performance was assessed by comparison against field measurements of latent heat flux over savannah vegetation consisting of bushes interspersed with a herbaceous understorey. The modelled results indicate that in dry conditions, a Penman-Monteith based single source model performs adequately when predicting the latent heat flux. However, the models with two sources demonstrate that the bushes and herbs have very different responses to local climate. In all the models, evaporation is highly sensitive to stomatal resistance, suggesting that a better understanding of stomatal response would improve the accuracy of the models.

Carbon Dioxide Effects on Crop Energy Balance: Testing ecosys with a Free‐Air CO2 Enrichment (FACE) Experiment

AbstractElevated CO2 concentrations (Ce) have been observed to decrease short‐term plant water use under controlled conditions by increasing stomatal resistance. The extent to which this decrease occurs over a growing season in the field is uncertain, however, because stomatal resistance is only one of many mechanisms that control water use. In this study, we tested the ecosystem simulation model ecosys, which reproduces an hourly energy balance through soil‐vegetation systems under defined atmospheric boundary conditions, using energy exchange data measured as part of the Free‐Air CO2 Enrichment (FACE) experiment at Ce = 550 vs. 370 µmol mol−1. The model reproduced reductions in measured upward latent heat fluxes that varied from −10 to +40 W m−2, depending on atmospheric conditions. In the model, the primary effect of elevated Ce on latent heat fluxes was through canopy stomatal conductance. This effect was largely offset by secondary effects through canopy temperature that enabled the model to reproduce measured changes in sensible heat fluxes. The total effect simulated by ecosys of Ce = 550 vs. 370 µmol mol−1 on evapotranspiration during the entire FACE experiment was a reduction of 7%. This reduction compares with one of 11% estimated from accumulated daily measurements of latent heat flux. In the model, the different effects of Ce on plant water use depend on atmosphere and soil boundary conditions, and are highly dynamic. Consequently the simulated Ce‐water use relationship is likely to be sitespecific. The use of models such as ecosys allows site‐specific boundary conditions to be considered in the study of Ce effects on plant growth and water use.

A comparison of bowen ratio and eddy correlation sensible and latent heat flux measurements above deciduous forest

Sensible and latent heat flux densities (H and ΛE) were measured above a mature, 18 m deciduous forest during July and August, 1988, using the Bowen ratio-energy balance (BREB) and eddy correlation (EC) methods. EC estimates ofH and ΛE underestimated day-time surface available energy by 11%. EC also partitioned available energy differently than BREB. forź/L<0.0, EC favouredH and BREB favoured ΛE.

Error analysis in estimating soil water balance of irrigated fields during the EFEDA experiment: 2. Spatial standpoint

In Part 1 of this paper, an error analysis for estimating the variance of volumetric water content, soil water storage and actual evapotranspiration from a local standpoint was presented. This second part extends the analysis to the case of spatial average values corresponding to a field scale. For a low number of sampling points, the location component related to the spatial variability is the major component of the total variance of the foregoing variables. Results are presented for the two maize fields described in Part 1 and comparison is made with values of evapotranspiration provided by atmospheric latent heat flux measurements for one of them. Finally, the determination of the calibration, instrument, integration and location components of the variance of a given variable leads to an estimation of the number of sampling points required to estimate field average values with a prescribed degree of precision. This number varies significantly depending on the variable considered — water content, water storage or evapotranspiration — and proved to be different for the two fields.